1. Field of the Invention
The present invention relates to an acousto-optic device which may be utilized for modulating, switching and scanning laser beams.
2. Description of Prior Art
There are available in the prior art acousto-optic (AO) devices that operate based on the diffraction of light by acoustic waves. The device, generally referred to as the AO Bragg cell, comprises an optically transparent medium onto which piezoelectric transducers are bonded. The transducer converts an input RF into a traveling acoustic wave in the medium. By varying the frequency and amplitude of the RF signal, the AO Bragg cell can be used to deflect and modulate an optical beam.
As an optical deflector, the AO Bragg cell scans an incoming laser beam into a range of angular positions or spots according to the frequency of the RF signal. In order to achieve large number of resolvable spots, the incident laser beam must be well-collimated with small angular divergence. The resolution or maximum number of resolvable spots of an AO deflector N is equal to the product of the frequency bandwidth .DELTA.f and time aperture .tau., i.e., N=.DELTA.f.tau.. The primary objective in the design of the AO deflector is to maximize bandwidth, .DELTA.f and time aperture, .tau..
The bandwidth of an AO deflector is the frequency range for efficient light diffraction and is a measure of the deflector speed. One technique of increasing the bandwidth of the deflector is based on AO diffraction in a birefringent crystal. By choosing acoustic wavevector to be tangential to the locus of the diffracted light wavevector efficient AO diffraction is obtainable for a wide range of acoustic frequencies. This method of increasing deflector bandwidth is referred to as tangential phase matching (TPM). Birefringent deflectors operated at TPM is disclosed in a paper entitled, "Continuous Deflection of Laser Beams" appearing on pages 48-51 in the January, 1967 issue of Applied Physics Letters.
An alternative technique for increasing the bandwidth of AO deflector is the use of acoustic beam steering with a phased array of transducers. The simplest phased array employs fixed phase difference of 180 degrees between alternate transducer elements in a planar configuration. By selecting a proper inter-element spacing the acoustic beam can be steered to track the phase matching condition over a larger frequency range, thereby increasing the bandwidth of the AO deflector. From the wavevector construction, the acoustic wavevector is shown to be tangential to the locus of the diffracted light vector. Thus, by using phased array transducers, it is possible to realize the TPM condition. For isotropic AO diffraction, the inter-element spacing s of the phased array must be equal to the characteristic length L.sub.o =n.LAMBDA..sup.2.sub.o /.lambda..sub.o where n is the refractive index, .LAMBDA..sub.o is the acoustic wavelength at center frequency and .lambda..sub.o is the optical wavelength.
The planar transducer array has two radiation lobes, thus half of the acoustic power is wasted. A more efficient use of the acoustic power has been demonstrated using a stepped phased array where the height of each step in the phased array is equal to .LAMBDA..sub.1 /2. The phased array is blazed so that the beam steering angle from the transducer plane is zero at the reference acoustic wavelength .LAMBDA..sub.1. This results in a single lobe and an accompanying increase in efficiency. Wideband AO deflectors using planar and stepped phased arrays were described in an article by Korpel et al entitled, "A Television Display Using Acoustic Deflection and Modulation of Coherent Light" appearing on pages 1667-1675 in the October 1967 issue of Applied Optics, and another article by E. I. Gordon entitled, "A Review of Acousto-Optical Deflection and Modulation of Coherent Light" appearing on pages 325-335 of the same issue of Applied Optics. The stepped array AO deflector was also disclosed in U.S. Pat. No. 3,493,759. Since the stepped phased array transducers are difficult to implement in practice, simpler fabrication techniques have been proposed. These are disclosed in U.S. Pat. No. 4,381,887, entitled, "Simplified Acousto-Optic Deflector using Electronic Delays," and U.S. Pat. No. 4,671,620, entitled, "Phased-Array Acousto-Optic Bragg Cell".
It is possible to achieve tangential phase matching by combining phased array transducers and birefringent diffraction. The net effect is to shift the acoustic frequency for tangential phase matching. The phased array birefringent deflector is described in the following articles: "Birefringent Phased Array Bragg Cells," 1985 IEEE Ultrasonics Symposium Proceedings, pages 381-384 and "Generalized Phased Array Bragg Interaction in Anisotropic Crystals," 1991 Proceedings of SPIE, Vol. 1476, pages 178-179.
The time aperture of an AO deflector is equal to the acoustic transit time across the optical aperture. In some efficient AO materials such as TeO.sub.2, the acoustic propagation is highly anisotropic. The acoustic energy flow is along the group velocity direction and is in general noncollinear with the acoustic phase velocity direction. The acoustic energy walks off from the acoustic wavevector and thus the maximum time aperture obtainable is limited.
To realize a large deflector resolution, it is desireable to realize a wide bandwidth, meanwhile overcoming the limitation of time aperture.
The AO Bragg cell can also act as a laser beam modulator that provides amplitude or phase modulations. In this case, a large temporal modulation bandwidth is desired. In the case of collimated incident beam, an AO modulator can realize a large frequency bandwidth by operating at TPM condition. However, the spectral components of the diffracted light is spread over a range of angular directions and will not mix collinearly at the detector to realize the desired modulation bandwidth. In the reverse case of focused beam optics, light diffraction is inefficient since the phase matching condition will be satisfied only for a narrow range of incident light directions. Thus, the AO modulator requires large input and large output angular bandwidths. Until now it has not been possible to satisfy this dual requirement.
It is possible to obtain large angular aperture by utilizing birefringent AO diffractions. The acoustic wavevector in a birefringent diffraction is properly chosen so that the tangents to the loci of incident and diffracted light wavevectors are substantially parallel. The phase matching becomes relatively insensitive to the direction of incident light, a condition known as non-critical phase matching (NPM). However, this type of interaction geometry generally results in narrow bandwidth and has been used in another type of AO device known as the acousto-optic tunable filter, for filtering of light. The type of AOTF operated at NPM condition is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture," appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter."
Recent development of AO devices has been focused on integrated optic or guided wave structure, i.e., the interaction between surface acoustic waves (SAW) and guided optical waves. The use of phased array transducer for increasing the bandwidth of the guided wave AO deflector has been discussed in an article entitled, "Efficient Wideband Guided-Wave Acousto-Optic Bragg Diffraction Using Phased-Surface Array in LiNbO.sub.3 Waveguide," Appl. Opt., Vol. 16, pp. 1297-1304, May, 1977. A different method using two titled SAW transducers in a guided wave AO deflector is described in U.S. Pat. No. 4,027,946, entitled, "Acousto-Optic Guided Light Beam Device." Since the fractional bandwidth of a SAW transducer is small, the use of two transducer of staggered frequencies provides the AO deflector with a wider acoustic bandwidth.